Current golf balls have dimples in their surface. For the dimple distribution, regular octahedral, regular icosahedral and regular polyhedral arrangements are generally employed. A spherical surface is divided as a regular polyhedron to define surface units, typically triangular surface units, where dimples are arranged. These arrangements are effective in enabling easy dimple design and mold manufacture and achieving good aerodynamic isotropy.
These arrangements, however, have the following problem. In the respective surface units, dimples are arranged in the same pattern, that is, the same pattern is repeated. The patterns are regular so that the dimples and the lands between dimples (i.e., surface areas where no dimples are formed) are arranged in a regular manner. In one example, dimples and lands are arranged in a row. A golf ball having dimples repetitively arranged in the same pattern exhibit different aerodynamic properties depending on the axis of rotation which varies with a particular point of impact, leading to variations of distance. The dimple arrangement based on the above-described units encounters a limit in closely distributing dimples and is hardly regarded as exerting maximum aerodynamic performance.
More particularly, it was found that the dimple arrangement based on the above-described units has the problem that aerodynamic properties largely differ whether or not the boundary between the units is coincident with the direction of rotation. This is because the units are large. The difference of aerodynamic properties is very significant between hemispheres which are regarded as the maximum unit in the prior art arrangement method. The difference of aerodynamic properties between equatorial rotation and longitudinal rotation is large enough to be perceivable in an actual hitting test and demonstrated by the measurement results of a swing machine test.
A number of approaches have been proposed for eliminating such a difference of aerodynamic properties, but none of them have given a sweeping solution. Since all these approaches are halfway measures to correct the difference while maintaining the arrangement units unchanged, they fail to significantly improve aerodynamic isotropy.